The pH and mineral content of water are the result of interactions between the soil beneath a pond and the water used to fill it. Clay soils are often acidic. Because ponds are commonly constructed on these soils, the effect on water quality can be significant. Ponds with acidic bottom soils that are filled with poorly mineralized water characteristically have low alkalinity and hardness. When total alkalinity and hardness are below 20 mg/L (as CaCO3) pH and productivity are usually reduced. Alkalinity concentrations below 20 mg/L often lead to large swings in daily pH values, which stress aquatic animals. Acidic soils contain high concentrations of hydrogen ions and/or aluminum relative to the concentrations of calcium and magnesium, which are important minerals for good water quality. The acidity of pond soils can be neutralized and the productivity of the pond improved by liming. “Liming” refers to the application of various acid-neutralizing compounds of calcium, or calcium and magnesium. Liming ponds has three important benefits. Liming may enhance the effect of fertilization. Liming helps prevent wide swings in pH. Liming also adds calcium and magnesium, which are important in animal physiology.